This invention relates to a method including producing, on the surface of a substrate, a stencil from a photosensitive layer comprising a dichromate-sensitized PVA (polyvinyl alcohol) and fluorescein-type dye. The method may be included in a method for etching flat apertured masks for use in color television picture tubes and for etching other precision-etched products.
The preparation of flat apertured masks by etching a stencilled metal sheet has been described previously; for example, in U.S. Pat. Nos. 4,061,529 issued Dec. 6, 1977 to A. Goldman, 4,126,510 issued Nov. 21, 1978 to J. J. Moscony et al., and 4,389,279 issued June 21, 1983 to D. M. Weber et al. In a typical industrial process for producing apertured masks, a substrate in the form of a thin metal sheet of cold-rolled steel, for example, is coated on both major surfaces with a liquid composition of a binder, a dichromate photosensitizer for the binder and water. The coatings are heated at temperatures in the range of about 120.degree. to 200.degree. C. to produce dry photosensitive coatings. After a storage period, the coatings are exposed to light images of actinic radiation whereby the exposed portions of the coatings harden; that is, become substantially insoluble, in water. The exposed coatings are sprayed with water whereby the still-soluble portions of the coatings are removed leaving the insolubilized portions in place, thereby producing stencils on the surfaces. The stencils are baked at temperatures in the range of about 200.degree. to 400.degree. C. to improve their resistance to attack by the acid etchant. Then, the metal sheet is etched through the stencils to produce the desired flat masks. The stencils are removed from the etched sheet, and then the flat masks are cut or torn from the sheet.
A number of special problems arise when dichromated PVA is used as the photosensitive coating. During the initial drying step, the photosensitive coatings are more or less insolubilized by action of the heat used for drying. This "heat-hardening" effect varies and is difficult to control. Also, during the storage periods before and after exposure to light images, the photosensitive coatings more or less insolubilize without light or heat being applied. This "dark-hardening" effect varies and also is difficult to control. The effect of heat-hardening and dark-hardening can be reduced by reducing the concentrations of dichromate sensitizer in the coatings, but this reduction also reduces the photosensitivities of the coatings, and reduces the resistance to attack by the acid etchant of the stencils produced therefrom.
Another problem, which occurs after etching the sheet, is the incomplete removal of the PVA stencils from the etched sheet. None of the expedients suggested in U.S. Pat. Nos. 4,208,242 issued June 17, 1980 to P. Zampiello, 4,339,528 and 4,339,529 both issued on July 18, 1982 to A. Goldman and 4,401,508 issued Aug. 30, 1983 to P. M. Ritt have proven to be adequate for industrial use.